(1) Field of the Invention
The present invention relates to an electroconductive metal oxide sintered body, a process for the preparation thereof, and the use thereof. More particularly, it relates to a zinc oxide sintered body which can be formed into a low-resistance, transparent, electroconductive film by the sputtering method, especially by the direct current magnetron sputtering method, a sputtering target, and a process for the preparation thereof.
(2) Description of the Related Art
There is now an increasing demand for a transparent electroconductive metal oxide film usable as a transparent electrode of a solar cell or display device or as an antistatic electroconductive coating.
A transparent electroconductive film of an electroconductive metal oxide is usually formed by sputtering a metal oxide, and as the metal oxide, there are industrially used indium oxide (ITO) which has been doped with tin as the hetero atom and tin oxide (TAO) which has been doped with antimony as the hetero atom.
ITO has a high transparency and a low-resistance film can be prepared from ITO, but ITO is economically disadvantageous because indium is expensive, and the field of application for ITO is restricted because ITO is chemically unstable. On the other hand, TAO is cheap and is relatively stable chemically, but since the resistance thereof is high, ATO is not completely satisfactory as the sputtering material.
It has been reported that a transparent electroconductive film having a low resistance comparable to that of ITO, and a high transparency, is obtained by sputtering zinc oxide which has been doped with aluminum or the like as the hetero atom [J. Appl. Phys., 55 (4), 15, February 1988, page 1029].
Since zinc oxide is cheap and chemically stable and has a good transparency and electroconductivity, zinc oxide is a satisfactory transparent electroconductive material that can be used instead of ITO and the like.
Nevertheless, the conventional hetero atom-containing zinc oxide sintered body used for the sputtering has a low density such as a sintered density lower than 5 g/cm.sup.3 and a high resistance such as a resistivity higher than several k.OMEGA.-cm. Since the density of this sintered body is low, the mechanical strength is low and the sintered body is very easily cracked. Moreover, since the electric resistance of this sintered body is high, the applicable sputtering method is limited, i.e., only the high-frequency sputtering method can be employed as the sputtering method, and the industrial direct current sputtering method cannot employed. Furthermore, in the case of the high-frequency sputtering method, if the substrate temperature is elevated, a problem arises of a lowering of the electroconductivity of the obtained zinc oxide transparent electroconductive film. Therefore, according to the conventional methods, a highly transparent electroconductive film cannot be obtained from the zinc oxide sintered body. If a sintered body having a high resistance and a low density is forcibly subjected to direct current sputtering, an applicable electric power is very small and the discharge is very unstable, and if a large electric power is forcibly applied, problems arise of a cracking of the target and a peeling and separation of the target from a backing plate.
The use of zinc oxide as a resistance element has been long known, and it has been pointed out that the electroconductivity of zinc oxide depends on the resistance of the grain boundary. Accordingly, it is known that, where zinc oxide is used as a resistance element, sintered particles are grown to a large grain size by elevating the sintering temperature and a sintered body having a relatively low resistance is obtained [The Journal of Association of Ceramics, 82, (5) page 271, 1974].
Nevertheless, the sintering property is reduced by the hetero atom incorporated to increase the electroconductivity, and an electroconductive zinc oxide sintered body having a high density such as a sintered density of at least 5 g/cm.sup.3 has not been reported.
Moreover, in the case of zinc oxide in which a hetero atom has been incorporated, a problem arises in that growth of sintered particles and a melting and fusion bonding of grain boundaries of the sintered particles are advanced at the sintering step, and sealed pores, i.e., closed cells, insulated from the outside are left in the interior of the sintered body.
A fusion bonding of the grain boundaries of sintered particles causes a reduction of the electroconductivity of the sintered body, and a gas is generally contained in the pores left in the interior of the sintered body. Accordingly, when the sputtering is carried out by using this sintered body as the target, as the target is consumed, the pores are communicated with the outside and the gas contained in the pores is irregularly discharged to disturb the formation of the film. Especially, the zinc oxide transparent electroconductive film is sensitive to oxygen contained in the film-forming atmosphere, and the film is adversely influenced by the gas discharged from the interior of the zinc oxide sintered body.
In general, sputtering is carried out under a low pressure of from about 0.1 Pa to about 0.5 Pa, but if the gas is discharged from the above-mentioned pores, the inner pressure of the sputtering apparatus is elevated to a level higher than 1 Pa and control of the pressure becomes very difficult.
As apparent from the foregoing description, an electroconductive zinc oxide sintered body having a high density and a low resistance and able to be satisfactorily used as a direct current sputtering electrode for forming a zinc oxide transparent electroconductive film has not been proposed.
The inventors made investigations into electroconductive zinc oxide sputtering targets, and as a result, found that a zinc oxide sintered to be used as the sputtering target must have a high density and a low resistance and must not substantially contain in the interior thereof closed cells insulated from the outside. It also was found that this sintered body target can be obtained by sintering a mixture of an oxide having a positive valency of at least 3 and zinc oxide at a high temperature in the presence of water, to control the sintered density of zinc oxide and to effect a fusion-bonding grain of the boundaries of sintered particles. The inventors have completed the present invention based on these findings.
In one aspect of the present invention, there is provided an electroconductive zinc oxide sintered body which is composed of sintered particles fusion-bonded substantially only in the grain boundaries thereof; which has a sintered density of from 5 to 5.5 g/cm.sup.3 ; and which does not substantially contain closed cells insulated from the outside in the interior of the sintered body.
In another aspect of the present invention, there is provided a process for the preparation of an electroconductive zinc oxide sintered body, which comprises sintering a mixture of an oxide of an element having a positive valency of at least 3 and zinc oxide in the presence of water at a temperature of at least 1,300.degree. C.
In a further aspect of the present invention, there is provided a sputtering target composed of the above-mentioned electroconductive zinc oxide sintered body.